Subject:
A Spanish Dancer (Hexabranchus sanguineus) showing its best moves on a Mauritian reef
The Spanish Dancer, a large nudibranch found in the tropical waters and the coral reefs of the Indo-Pacific region, is known for its reddish colour and its undulating swimming movements, which resembles the undulations made by a Spanish flamenco dancer.
Flamenco is a traditional south Spanish dance typically accompanied by acoustic guitar and castanets. The Spanish Dancer nudibranch can grow up to a stunning length of 60cm/24inch, making it one of the biggest nudibranchs in the world, although it is more commonly encountered at a much smaller size.
This nocturnal nudibranch comes in various hues of red, including dessert-icing pink, egg-yolk yellow, sunset orange, and blood red, with its body often speckled with small whitish dots or speckles. Its scientific latin name Hexabranchus sanguineus literally means "blood-coloured six-gills”.
When disturbed, the Spanish Dancer will make a near vertical escape by undulating its body and generating propulsion through the wave-like movement of its wing-like mantle just like in this short underwater video. After a brief period of swimming upwards in the water column, the Spanish Dancer will stop its undulation-like movements and slowly sink to the ocean's floor, where it will crawl in a typical nudibranch's way; by pushing itself forward with its flat flexible foot through a series of muscular contractions.
Although the Spanish Dancer is a remarkable swimmer, it is not the only gastropod with swimming abilities. Some seahares (genus Aplysia) can easily outperform the Spanish Dancer in terms of swimming speed and endurance, thanks to their parapodia, wing-like structures on both sides of their body. A swimming seahare can look like a vampire or bat-like creature flying right out of a fairy tale book.
Most nudibranchs with swimming abilities use this skill to avoid predation rather than as a regular form of locomotion. In most sea slugs with this special form of locomotion, the swimming is usually nothing more than an energetically lateral bending back and forth of the animal's body. Observing different swimming nudibranchs leads to the conclusion that swimming is, for most nudibranch species, not a very graceful means of locomotion but just a frantic attempt to escape imminent predation.
Technique:
Underwater videographers usually anticipate the path of any underwater critter to keep the animal centred in their viewfinder. However, filming a frantic moving nudibranch in the water column is more challenging than anticipated. The 20cm/8inch long nudibranch in this underwater videoclip had a rather unpredictable flightpath, slightly changing direction and speed every three seconds.
It is crucial to keep the distance between the camera's lens and the subject as constant as possible, and ensure that the subject doesn't fill the entire frame so that you can easily centre it and adjust its size during the editing process using keyframes.
Also, position yourself in a way that provides the best vantage point to capture the graceful undulations of the Spanish Dancer. To avoid background distractions in the image, it is recommended to film slightly upward to obtain a uniformly blue background.
For an other video and vlog post about the Spanish Dancer please go to our vlog post 118 or click on this link: https://www.beyondscuba.com/post/spanish-dancer-ll
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